────────────┬────────────┬─────────┬───────────────────────────────── Velocity in │ Diameter of│ Head. │ Ft. per Sec.│ Pipe. │ │ ────────────┼────────────┼─────────┤ │ in. │ │ 84·8 │ 6 │ ·186 │ } 37·7 │ 9 │ ·02442 │ } To pass 100 ft. per min. these 21·2 │ 12 │ ·00579 │ } figures must be divided by 9·4 │ 18 │ ·000763 │ } 100. To pass 10,000 ft. they 5·3 │ 24 │ ·000181 │ } must be multiplied by 100. 3·4 │ 30 │ ·0000593│ } ────────────┴────────────┴─────────┴───────────────────────────────── To calculate the head required for a long pipe, multiply the head given by the table by the length in yards. The air passed by square pipes of the same diameters will be 1·273 times greater with the same heads. To be added to the pressure required to overcome friction is that needed to force the air out at the end of the pipe. This varies with the shape of the tube, &c., but for our purpose may be taken as given in the following table:-- Table V. Approximate Pressure needed to force Air out of a Pipe with a Velocity of-- Ft. per Sec. Head in Inches, Water. 84·8 1·8 37·7 0·36 21·2 0·13 9·4 0·02 5·3 under 0·01 3·4 under 0·005 Air-passages should be, as far as practicable, of uniform area throughout their length, as much velocity is lost in passing even from a smaller to a larger tube. Of course sharp bends must be avoided. CHAPTER XXIV. COMMERCE, STATISTICS, AND BIBLIOGRAPHY. =Skins.=--The trade in skins possesses no small importance. Many of the statistics relating to skins are collective, and not specific; these will be grouped under the heads of the respective countries, after all accessible details have been given upon each kind of skin. _Alligator._--In the Southern United States, notably Florida, the supply of alligator-skins amounts to many thousands annually, and the "farming" of the reptiles for their skins is even spoken of. The principal market for them is Europe, but no statistics of the trade are published. The alligators often attain a length of 18-20 ft. The hides are stripped off, and the belly and sides, the only portions fit for use, are packed in barrels in a strong brine, and shipped to the Northern tanner, who keeps them under treatment for 6-8 months, when they are ready to be cut up. So far the leather has been principally used in the manufacture of boots and shoes, for which it is especially adapted. _Armadillo._--The skins of this animal were exported from Brunei (Borneo) to Singapore to the value of 121 _dol._ (of 4_s._ 2_d._) in 1879. _Ass._--Hankow exported 2402-1/2 _piculs_ (of 133-1/2 lb.) of asses' skins in 1878, and 1068 _piculs_ in 1879.